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LM5149-Q1: IN48V OUT25V8A

kazutaka takayama
Prodigy 10 points
Part Number: LM5149-Q1
Other Parts Discussed in Thread: LM5149-LM25149DESIGN-CALC, LM5149

We are currently prototyping a power supply with an input of 48V and an output of 25V and 8A.
Set constants using LM5149-LM25149DESIGN-CALC.

I am aiming for stable operation with a simple circuit.

AEF (No Input Filter)
UVLO　35V-30V
DITHER Disabled
FPWM　(Tie PFM to GND)
External Compensation

I have inserted a 1kΩ and 0.1uF filter into ISNS+.
It operates stably at 150kHz when the load current is around 3A, but the oscillation frequency drops to about 20kHz when the load is less than 2A or more than 4A. The current amplitude increases, saturating the inductor and destroying the FET.
Could you please advise me on the cause of the frequency change?

  • 0
    TI__Genius 10760 points

    Hi,

    I think the cause is your layout. Noise goes into the controller will cause this problem. Could you upload your schematic and layout here? Or firstly, you can try to add a 10Ω gate resistor on the HS-FET to see if it gets better.

    Thank you. 

    BR,

    Shipeng

  • 0 in reply to Shipeng Cheng
    Prodigy 10 points

    Shipeng Cheng
    thank you.
    Attached is the circuit diagram and board layout.
    I designed the board with two layers, so I think the layout will be difficult.
    Adding a 330uF electrolytic capacitor to the output and inserting a filter to Isns+ made things a little better.
    Even if I shorted the ISNS+ input and eliminated the effect, the problem did not go away, so I thought there might be other causes, so I consulted with you.
    The AEF peripheral area is not mounted and the IC terminals are open, but is there any problem with the terminal processing?
    I'll try the gate resistor now.

  • 0 in reply to kazutaka takayama
    TI__Genius 10760 points

    Hi,

    There's a mistake of the MOSFET choose. LM5149 only have a 5V driver so a logic level MOSFET is suitable. ISC060N10NM6 can't work normally here so I recommend BSC070N10LS5.

    I think add a gate resistor is necessary in a 2-layer design. You can try to find a way to add a gate resistor for high-side MOSFET and I think things will go better.

    Thanks.

    BR,

    Shipeng

  • 0 in reply to Shipeng Cheng
    Prodigy 10 points

    Shipeng Cheng
    Thank you.

    As advised, I replaced it with logic level BSC070N10LS5.
    The situation has improved dramatically.
    thank you.